NAME
`Event::Distributor' - a simple in-process pub/sub mechanism
SYNOPSIS
use Event::Distributor;
my $dist = Event::Distributor->new;
$dist->declare_signal( "announce" );
$dist->subscribe_sync( announce => sub {
my ( $message ) = @_;
say $message;
});
$dist->subscribe_async( announce => sub {
my ( $message ) = @_;
return $async_http->POST( "http://server/message", $message );
});
$dist->fire_sync( announce => "Hello, world!" );
DESCRIPTION
Instances of this class provide a simple publish/subscribe mechanism
within a single process, for either synchronous or Future-based
asynchronous use.
A given instance has a set of named events. Subscribers are `CODE'
references attached to a named event. Publishers can declare the
existence of a named event, and then later invoke it by passing in
arguments, which are distributed to all of the subscribers of that named
event.
It is specifically *not* an error to request to subscribe an event that
has not yet been declared, in order to allow multiple modules of code to
be loaded and subscribe events the others publish, without introducing
loading order dependencies. An event only needs to be declared by the
time it is fired.
Natively all of the events provided by the distributor are
fully-asynchronous in nature. Each subscriber is expected to return a
`Future' instance which will indicate its completion; the results of
these are merged into a single future returned by the fire method
itself. However, to support synchronous or semi-synchronous programs
using it, both the observe and invoke methods also have a synchronous
variant. Note however, that this module does not provide any kind of
asynchronous detachment of synchronous functions; using the
`subscribe_sync' method to subscribe a long-running blocking function
will cause the `fire_*' methods to block until that method returns. To
achieve a truely-asynchronous experience the attached code will need to
use some kind of asynchronous event system.
This module is very-much a work-in-progress, and many ideas may still be
added or changed about it. It is the start of a concrete implementaion
of some of the ideas in my "Event-Reflexive Programming" series of blog
posts. See the TODO and SEE ALSO sections for more detail.
EVENTS
Each of the events known by a distributor has a name. Conceptually each
also has a type. Currently there are two types of event, a "signal", and
a "query".
A signal event simply informs subscribers that some event or condition
has occurred. Additional arguments can be passed from the invoker to the
subscribers, but subscriptions are not expected to return a meaningful
value, nor does firing this event return a value. All subscriber
functions are invoked sequentually and synchronously by a `fire_*'
method (though, of course, asynchronous subscribers synchronously return
a future instance, which allows them to continue working
asynchronously).
A query event invokes subscriber code expecting a successful return,
returning the first result that is successful. If a synchronous
subscriber returns a result, or if an asynchronous one returns a
successful immediate Future, then no further subscribers are invoked,
and that result is taken immediately. Any other pending Futures are then
cancelled.
METHODS
$distributor->declare_signal( $name )
Declares a new "signal" event of the given name.
$distributor->declare_signal( $name )
Declares a new "signal" event of the given name.
$distributor->subscribe_async( $name, $code )
Adds a new `CODE' reference to the list of subscribers for the named
event. This subscriber is expected to return a Future that will
eventually yield its result.
When invoked the code will be passed the distributor object itself and
the list of arguments, and is expected to return a `Future'.
$f = $code->( $distributor, @args )
$distributor->subscribe_sync( $name, $code )
Adds a new `CODE' reference to the list of subscribers for the named
event. This subscriber is expected to perform its work synchronously and
return its result immediately.
In non-blocking or asynchronous applications, this method should only be
used for simple subscribers which can immediately return having
completed their work. If the work is likely to take some time by
blocking on external factors, consider instead using the
`subscribe_async' method.
When invoked the code will be passed the distributor object itself and
the list of arguments.
$code->( $distributor, @args )
$f = $distributor->fire_async( $name, @args )
Invokes the named event, passing the arguments to the subscriber
functions. This function returns as soon as all the subscriber functions
have been invoked, returning a Future that will eventually complete when
all the futures returned by the subscriber functions have completed.
$distributor->fire_sync( $name, @args )
Invokes the named event, passing the arguments to the subscriber
functions. This function synchronously waits until all the subscriber
futures have completed, and will return once they have all done so.
Note that since this method calls the `get' method on the Future
instance returned by `fire_async', it is required that this either be an
immediate, or be some subclass that can actually perform the await
operation. This should be the case if it is provided by an event
framework or similar, or custom application logic.
TODO
Some of these ideas appear in the "Event-Reflexive Progamming" series of
blog posts, and may be suitable for implementation here. All of these
ideas are simply for consideration; there is no explicit promise that
any of these will actually be implemented.
* Anonymous signals. Move observer storage into the actual Signal
objects, use the overall `Event::Distributor' object largely as a
named-lookup store for them.
* Unsubscription from events.
* Define (or document the lack of) ordering between subscriptions of a
given event.
* Refine the failure-handling semantics of signals.
* Ability to invoke signals after the current one is finished, by
deferring the `fire' method. Should this be a new `fire_*' method,
or a property of the signal itself?
* More control over the semantics of value-returning events -
scatter/map/gather pattern.
* Sub-heirarchies of events.
* Subclasses for specific event frameworks (IO::Async).
* Subclasses (or other behaviours) for out-of-process event
serialisation and subscribers.
* Event parameter filtering mechanics - allows parametric heirarchies,
instrumentation logging, efficient out-of-process subscribers.
SEE ALSO
Event-Reflexive Programming
AUTHOR
Paul Evans